I got out of work a little early today, so I made a few measurements and headed to Lowes.
One piece of 4x8 plywood, Lowes gives you up to 5 cuts for free which is pretty good. So the top piece is 48x41 and it’s set 14 inches high to clear the toolboxes, which it does by about half an inch.
Tomorrow I need to cut the pieces to support the sides. It won’t be a full box, because of the wheel wells, and I’ll be left with some little side compartments...I don’t know if I’ll hinge in some covers or not, but that’s not high on the list of priorities. Screwing it together and carpeting it is basically all that’s left.